Dusk Over Dawn O2 Asymmetry in Europa's Near-Surface Atmosphere

©NASA

Europa's Surface

The evolution of Europa's water-product exosphere over its 85-hour day, based on current models, has not been shown to exhibit any diurnal asymmetries. Here we simulate Europa's exosphere using a 3-D Monte Carlo routine including, for the first time, the role of Europa's rotation on the evolution of exospheric molecules throughout the orbit.

We focus on O2, sputtered by a trailing hemisphere source with a temperature-dependence under isotropic plasma conditions. We find that the O2 component, while global, is not homogenous in Europa local time. Rather, the O2 accumulates at the dusk hemisphere. When rotation is explicitly excluded, no diurnal asymmetries exist. We find that the assumed thermal-dependence on the O2 source is critical for a diurnal asymmetry: the diurnal surface temperature profile is imprinted on to the near-surface O2 atmosphere, due to the small hop times of the non-adsorbing O2 effectively rotating with Europa.

Altogether, our simulations conclude that the dusk-over-dawn asymmetry is driven by Europa's day-night O2 cycle synchronized with Europa's orbital period based on our model assumptions on O2 production and loss. This conclusion is in agreement with the recent understanding that a non-adsorbing, rotating O2 source peaking at noon will naturally accumulate from dawn-to-dusk, should the O2 lifetime be sufficiently long compared to the orbital period. Lastly we compare hemispherically-averaged dusk-over-dawn ratios to the recently observed oxygen emission data by the Hubble Space Telescope. We find that while the simulations are globally consistent with brighter oxygen emission at dusk than at dawn, the orbital evolution of the asymmetries in our simulations can be improved by ameliorating the O2 source & loss rates, and possibly adsorption onto the regolith.

Apurva V. Oza, Francois Leblanc, Robert E. Johnson, Carl Schmidt, Ludivine Leclercq, Timothy A. Cassidy, Jean-Yves Chaufray
(Submitted on 27 Apr 2018)

Comments: 15 pages, 7 figures, Submitted to PSS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1804.10582 [astro-ph.EP] (or arXiv:1804.10582v1 [astro-ph.EP] for this version)
Submission history
From: Apurva Oza
[v1] Fri, 27 Apr 2018 16:32:01 GMT (15574kb,D)
https://arxiv.org/abs/1804.10582
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